Not applicable.
The present invention generally relates to metal forming methods and tooling used therefor. More particularly, this invention relates to a coating for tooling used in superplastic forming (SPF), quick plastic forming (QPF), and related forming methods, and to a forming process made possible with the coating as a result of the coating reducing wear and sticking between the tooling and the article formed thereon to the extent that the use of lubricants can be significantly reduced or eliminated.
The term xe2x80x9csuperplasticityxe2x80x9d is used to denote the exceptional ductility that certain metal alloys can exhibit when deformed under proper conditions, the process of which is known as superplastic forming (SPE). Typical examples are titanium and aluminum alloys capable of being deformed to elongations in excess of 100%. General conditions for superplasticity include very fine grain size (e.g., less than ten micrometers), high temperatures (e.g., greater than one-half of the absolute melting temperature of the alloy) and a controlled strain rate (typically 10xe2x88x924 to 10xe2x88x923 sxe2x88x921). A related forming process referred to as xe2x80x9cquick plastic formingxe2x80x9d (QPF) is disclosed in commonly-assigned U.S. Pat. No. 6,253,588, and enables more rapid strain rates (above 10xe2x88x923 sxe2x88x921) to provide a more economical and practical process for mass-producing parts.
SPF and QPF methods typically involve blow-forming a sheet of the desired alloy into a sculptured ferrous tool that is heated to an appropriate forming temperature, yielding a deformed workpiece that is in intimate contact with the tool. The workpiece must release cleanly from the tool in order to maintain its integrity, such as dimensional accuracy and surface finish, particularly if a Class A type surface (Ra below 50 microinches (1.27 micrometers)) is desired, as is the case with automobile body panels. However, the intimate contact that occurs between the workpiece and tool during an SPF/QPF process leads to the action of interatomic forces (adhesion, friction) to the extent that workpiece release and quality are processing issues with SPF/QPF. Workpiece adhesion leads to galling patterns appearing on the finished workpiece surfaces, and forcible separation of a workpiece from an SPF tool can distort the workpiece beyond its allowable dimensions. Under such conditions, the use of a robotic material handling system would be very difficult to implement, eliminating the possibility of having a large-scale production process.
As a result, SPF/QPF tooling and/or the workpiece are coated with a lubricant or release agent, such as graphite or boron nitride, to prevent sticking and bonding of the workpiece to the tooling. An alternative is an improved SPF/QPF release agent comprising magnesium hydroxide (Mg(OH)2), disclosed in commonly-assigned U.S. Pat. No. 5,819,572 to Krajewski. While suitable for many applications, lubricants can have an adverse effect on the final surface characteristics of a superplastically formed workpiece. As an example, the surface characteristics of an aluminum part formed by SPF on a ferrous tool strongly depend on the conditions of the tool surface and the amount of lubricant applied. In addition to machining marks, scratches and excessive roughness of the tool surface, any lubricant buildup on the tool will be reproduced on the workpiece surface during the forming process, and potentially prevent the production of a Class A type surface. Excess lubricant is also associated with necking and eventual breaks in a workpiece due to excessive slippage between the workpiece and tool. On the other hand, insufficient lubricant is a common cause of breaks, splits and incomplete forming of workpiece details.
In view of the above, it would be desirable if an improved SPF/QPF tooling and process were available that was less prone to workpiece sticking and therefore workpiece distortion and dimensional inaccuracies, while also enabling the production of parts with excellent surface characteristics.
The present invention is directed to a coating for SPF/QPF tooling and an SPF/QPF process made possible with the coating. The coating reduces sticking and wear between the tooling and the workpiece formed thereon to the extent that the use of lubricants can be significantly reduced or eliminated.
The coating of this invention defines the outer surface of an SPF/QPF tool, and consists essentially of either a tungsten carbide cermet or a chromium carbide cermet. The coating preferably comprises a metal matrix containing tungsten carbide or chromium carbide particles having a particle size of not more than 0.1 micrometer, and is preferably prepared to have an average surface roughness (Ra) of not higher than 0.3 micrometer. Under certain conditions, an SPF/QPF process that makes use of a tool whose forming surface is provided with the coating of this invention can be performed without any lubricant on the forming surface or workpiece. As with known SPF and QPF processes, such a process will be carried out at relative high temperatures, e.g., greater than one-half of the absolute melting temperature of the workpiece.
According to the present invention, tooling with tungsten carbide cermet or chromium carbide cermet coatings of this invention have been shown to be more resistant to wear than conventional lubricated SPF/QPF tooling, such that more workpieces can be formed with the tooling without refinishing the tooling forming surface. As a result, tooling of this invention requires less maintenance, and production cost and downtime are reduced. If a lubricant or release agent is used with the coating of this invention, more workpieces can be formed without cleaning the tooling forming surface than with conventional SPF/QPF tooling. If the coating is prepared to be sufficiently effective to reduce or eliminate the need for a lubricant or release agent, the process cycle time can be significantly decreased and the likelihood that the lubricant will degrade the workpiece properties is reduced. Finally, workpieces have been shown to release more readily and cleanly from SPF/QPF tooling protected with the coating of this invention, enabling the mass production of workpieces with Class A type surfaces.
Other objects and advantages of this invention will be better appreciated from the following detailed description.